1. Field of the Invention
This invention relates to a process for the production of a high density and high strength silicon carbide sintered body, and more particularly to a process for producing a silicon carbide sintered body by charging a sintering raw material consisting mainly of fine silicon carbide powder and a sintering aid into a dispersion medium to form a uniform suspension, spray freezing the suspension to form a granular frozen body, freeze drying the frozen body for the sublimation of the dispersion medium to obtain a powdery or granular dried mixture, shaping the dried mixture into a green body of an optional form and then sintering the green body without using any pressing techniques.
2. Description of the Prior Art
Silicon carbide has excellent chemical and physical properties, and is suitable for high temperature structural applications used under severe conditions, such as components for gas turbines, heat exchangers at higher temperature and the like.
Heretofore, there have been widely known a hot pressing process, a reaction sintering process and the like. However in the hot pressing process it is very difficult to produce a sintered body of a complicated form and also the productivity is low. In the reaction sintering process, it is very difficult to produce a high strength sintered body and also the heat strength at elevated temperature zone deteriorates due to the presence of a great amount of free silicon.
Since silicon carbide is a material which is difficult to sinter, it has hitherto been considered that it is very difficult to apply a process usually used in the production of oxide ceramics, wherein a green article shaped at room temperature is sintered without pressing, to the sintering of silicon carbide. However, there has recently been proposed a process for sintering silicon carbide without pressing, wherein a powder mixture of silicon carbide, boron-containing additive and carbonaceous additive is shaped into a green body and then sintered in an inert atmosphere. For instance, U.S. Pat. No. 4,004,934 discloses that silicon carbide is mixed with a boron-containing additive (0.3-3.0 wt% of B) and a carbon-containing additive (0.1-1.0 wt% of C) and shaped into a green body and then the resulting green body is sintered at a temperature of 1,900.degree.-2,100.degree. C. in an inert atmosphere without pressing to produce a silicon carbide sintered body having a density corresponding to at least 85% of theoretical density.
In the above pressureless sintering process, the boron-containing additive and carbonaceous additive are mainly used as a sintering aid, but a beryllium- or aluminum-containing additive may further be used. The addition of the boron-containing additive as a sintering aid is due to the fact that when boron is existent in each contact point between silicon carbide particles in the sintering, a tacky layer of boron is formed on the surface of the silicon carbide particle, resulting in a uniform advance of shrinkage. While, the carbonaceous additive is mainly added for reducing a silica film, which always covers the surface of the silicon carbide particles at room temperature and obstructs the self-sintering of silicon carbide, with carbon to enhance the sinterability of silicon carbide, and for suppressing exaggerated grain growth. Therefore, it is advantageous that the aforementioned sintering aids are uniformly dispersed into the fine silicon carbide powders.
As a means for the formation of a green shaped body in the ceramic pressureless sintering process, there have hitherto been known various methods such as slurry cast molding, extrusion molding, lathe molding, rocking molding, dry pressure molding and the like. These shaping methods can be largely divided into wet method and dry method in accordance with the liquid content in the shaping. The invention is mainly concerned with a process for the production of a high density and high strength silicon carbide sintered body by the dry shaping method.
In the pressureless sintering process, it is expected to obtain a silicon carbide sintered body having a higher strength in view of properties inherent to its silicon carbide. Among the conventionally known silicon carbide sintered bodies, however, a high strength sintered body is particularly produced only by starting from very expensive .beta.-type silicon carbide obtained by thermal decomposition of high molecular weight organosilicon compound or silicon carbide synthesized by a very special production method, because it is very difficult to industrially produce a high strength pressureless-sintered body of silicon carbide by starting from silicon carbide synthesized from silica and carbon in the usual manner.
The inventor has made various studies in order to clear up the cause of deteriorating the properties, particularly the mechanical strength of the silicon carbide pressureless-sintered body produced by using fine powders of silicon carbide synthesized from silica and carbon in the usual manner as a starting material, and as a result it has been concluded that the cause of deteriorating the properties, particularly the mechanical strength of the above pressureless-sintered body is due to the following mechanism.
That is, the sintering raw material used in the production of silicon carbide pressureless-sintered body is significant to be a fine powder of silicon carbide containing a uniformly dispersed sintering aid as previously mentioned. However, since the silicon carbide fine powder or the sintering aid is very strong in the agglomeration, it is difficult to obtain a mixture of these materials at a thoroughly uniform dispersed state without agglomerating. Therefore, a mixture of silicon carbide fine powder and sintering aid containing partly agglomerated particles has hitherto been used as a sintering raw material. As a result, when such a mixture is used to produce a silicon carbide pressureless-sintered body, the effect of the sintering aid is non-uniform in the sintering, which produces a microscopical scattering in the properties of the finally sintered body such as density, crystal grain size and the like, whereby the properties, particularly the mechanical strength of the sintered body, is considerably deteriorated.
Furthermore, since the aforementioned sintering raw material has a very fine particle size and has poor fluidity and moldability, when said raw material is shaped into a green body by the dry shaping method, it is difficult to uniformly charge into a mold under pressure, so that the scattering arises in the bulk density of the green body or the pressure distribution at the shaping, or defects due to the bridging are apt to occur in the green body. Such defects induced in the shaping remain in the sintered body after the sintering, which results in the considerable deterioration of the properties, particularly mechanical strength of the sintered body.
Now, there have hitherto been reported various method of preparing a sintering raw material composed of silicon carbide fine powder and sintering aid.
In U.S. Pat. No. 4,004,934, for instance, there is disclosed a method of preparing the sintering raw material wherein silicon carbide fine powder is dispersed into a solution of oleic acid and aluminum stearate in benzene and ball milled to yield a slurry and thereafter the resulting slurry is sieved and freeze dried to obtain a friable cake, which is then broken up and sieved. According to this method, however, a means for preventing the agglomeration is not considered at the stages after the ball milling treatment capable of giving a strong shearing force, so that a part of the silicon carbide fine powder and sintering aid is apt to be segregated by agglomeration until the freezing of the slurry. Moreover, the sintering raw material is produced by breaking up the friable cake and sieving it as mentioned above, so that the resulting granules are relatively angular resulting in a poor fluidity. As a result, it is difficult to uniformly charge these granules into a mold.
Furthermore, U.S. Pat. No. 4,041,117 discloses a method wherein a mixture of silicon carbide fine powder and sintering aid is ball milled in a solution of benzene containing polyethylene glycol to yield a slurry, which is then spray-dried. According to this method, however, a means for preventing the agglomeration is also not considered at the stages after the ball milling treatment capable of giving a strong shearing force likewise the case of U.S. Pat. No. 4,004,934, so that a part of the silicon carbide fine powder and sintering aid is segregated by selective agglomeration until the drying of the slurry, or the sintering aid is apt to be segregated with the shift of the dispersion medium in the drying. Moreover, owing to the spray drying of the slurry, the sintering raw material having a uniform particle size distribution and a round granular form can easily be produced in large quantities, but the apparent granule density becomes higher due to the shrinkage in the drying. As a result, the resulting granules are apt to be rigid, which results in the deterioration of the press moldability. The term "apparent granule density" used herein means a weight of granules per unit bulk volume, and the term "bulk volume" means a volume including a solid content occupied in the granules and an inner space.
As mentioned above, the conventionally known methods of preparing the sintering raw material have many drawbacks.